chore: add synchronousTransformIterator

Author: Jesse Wright

asynciterator.ts

@@ -527,7 +527,7 @@ export class AsyncIterator<T> extends EventEmitter {
     @returns {module:asynciterator.AsyncIterator} A new iterator with at most the given number of items
   */
   take(limit: number): AsyncIterator<T> {
-    return this.transform({ limit });
+    return new UntilIterator(this, limit);
   }
 
   /**
@@ -795,14 +795,12 @@ export function identity<S>(item: S): typeof item {
   return item;
 }
 
-
 /**
  An iterator that synchronously transforms every item from its source
  by applying a mapping function.
  @extends module:asynciterator.AsyncIterator
 */
-export class MappingIterator<S, D = S> extends AsyncIterator<D> {
-  protected readonly _map: MapFunction<S, D>;
+export abstract class SynchronousTransformIterator<S, D = S> extends AsyncIterator<D> {
   protected readonly _source: InternalSource<S>;
   protected readonly _destroySource: boolean;
 
@@ -811,11 +809,9 @@ export class MappingIterator<S, D = S> extends AsyncIterator<D> {
    */
   constructor(
     source: AsyncIterator<S>,
-    map: MapFunction<S, D> = identity as MapFunction<S, D>,
     options: SourcedIteratorOptions = {}
   ) {
     super();
-    this._map = map;
     this._source = ensureSourceAvailable(source);
     this._destroySource = options.destroySource !== false;
 
@@ -833,15 +829,16 @@ export class MappingIterator<S, D = S> extends AsyncIterator<D> {
     }
   }
 
+  protected abstract safeRead(): D | null;
+
   /* Tries to read the next item from the iterator. */
   read(): D | null {
     if (!this.done) {
       // Try to read an item that maps to a non-null value
       if (this._source.readable) {
-        let item: S | null, mapped: D | null;
-        while ((item = this._source.read()) !== null) {
-          if ((mapped = this._map(item)) !== null)
-            return mapped;
+        const item = this.safeRead();
+        if (item !== null) {
+          return item;
         }
       }
       this.readable = false;
@@ -865,6 +862,89 @@ export class MappingIterator<S, D = S> extends AsyncIterator<D> {
   }
 }
 
+export class MappingIterator<S, D = S> extends SynchronousTransformIterator<S, D> {
+  private _map: MapFunction<S, D>;
+  
+  constructor(
+    source: AsyncIterator<S>,
+    map: MapFunction<S, D> = identity as MapFunction<S, D>,
+    options: SourcedIteratorOptions = {}
+  ) {
+    super(source, options);
+    this._map = map;
+  }
+
+  safeRead() {
+    let item: S | null;
+    while ((item = this._source.read()) !== null) {
+      const mapped = this._map(item);
+      if (mapped !== null)
+        return mapped;
+    }
+    return null;
+  }
+
+  map<K>(map: MapFunction<D, K>, self?: any): AsyncIterator<K> {
+    return new CompositeMappingIterator(this._source, [this._map, bind(map, self)], this);
+  }
+}
+
+export class CompositeMappingIterator<S, D = S> extends SynchronousTransformIterator<S, D> {
+  constructor(
+    private root: AsyncIterator<S>,
+    private mappings: MapFunction<any, any>[] = [],
+    source: AsyncIterator<any>,
+    options: SourcedIteratorOptions = {},
+  ) {
+    super(source, options);
+  }
+
+  safeRead() {
+    // TODO: See if this is actually necessary
+    // A source should only be read from if readable is true
+    if (!this.root.readable) {
+      this.readable = false;
+      // TODO: See if this should be here
+      if (this.root.done)
+        this.close();
+      return null;
+    }
+
+    let mapped : any = null;
+    while (mapped === null && (mapped = this.root.read()) !== null) {
+      for (let i = 0; i < this.mappings.length; i++) {
+        mapped = this.mappings[i](mapped);
+        if (mapped === null)
+          break;
+      }
+    }
+    return mapped;
+  }
+
+  map<K>(map: MapFunction<D, K>, self?: any): AsyncIterator<K> {
+    return new CompositeMappingIterator(this.root, [...this.mappings, bind(map, self)], this);
+  }
+}
+
+export class UntilIterator<S> extends SynchronousTransformIterator<S> {
+  constructor(
+    source: AsyncIterator<S>,
+    private limit: number,
+    options: SourcedIteratorOptions = {}
+  ) {
+    super(source, options);
+  }
+
+  safeRead() {
+    if (this.limit <= 0) {
+      this.close();
+      return null;
+    }
+    this.limit -= 1;
+    return this._source.read();
+  }
+}
+
 // Validates an AsyncIterator for use as a source within another AsyncIterator
 function ensureSourceAvailable<S>(source?: AsyncIterator<S>, allowDestination = false) {
   if (!source || !isFunction(source.read) || !isFunction(source.on))

test/SimpleTransformIterator-test.js

@@ -1198,10 +1198,6 @@ describe('SimpleTransformIterator', () => {
           result.on('end', done);
         });
 
-        it('should be a SimpleTransformIterator', () => {
-          result.should.be.an.instanceof(SimpleTransformIterator);
-        });
-
         it('should take the given number of items', () => {
           items.should.deep.equal(['a', 'b', 'c']);
         });